This invention relates to a vibration type force detector for detecting a force acting on a string in the direction in which the string is tensioned by measuring the vibration frequency of the string. The invention utilizes the fact that the natural vibration frequency of a string depends upon the magnitude of the tension of the string. The force detector of the invention may be used as, for example, a load sensor in an electronic balance.
In force detectors of this type, while a force to be measured is applied to a string in the direction in which the string is tensioned, the string is vibrated laterally at its natural frequency by means of an oscillator, and the vibration frequency of the string is measured to detect the magnitude of the force. To measure the vibration frequency of the string, the number of the vibration may be counted for a certain period of time, or the cycle of the vibration may be measured.
The relation between the natural vibration frequency f of a string and the tension F thereof is expressed by the equation ##EQU1## wherein .rho. is the mass per unit length of the string and l is the length of the string. The equation means that the vibration frequency f changes in proportion to the square root of the tension F. Therefore, as the force F to be measured increases, the vibration frequency f of the string changes more and more gradually even when the force F changes the same amount, so that the amount of change of the number of vibration counted for the same period of time or the cycle of the vibration becomes smaller and smaller. This means that the greater the amount to be measured is in a measuring range, the lower the resolving power of the detector becomes.
If the vibration type force detector is used in an electronic balance, there will be an advantage that as the load to be measured increases, measurement may be made on a more and more coarse scale. If the tare of a sample to be measured, however, is great as compared with the contents thereof, the tare must be deducted for correct measurement of the contents of a small weight. In such a case the conventional vibration type force detectors are not suitable for use.
Japanese unexamined patent publication No. 48-30474 discloses a method of improving the resolving power uniformly within a measuring range by multiplying the frequency of the pulses produced in synchronism with the vibration of the string and counting the multiplied pulses within a predetermined period of time. Even with this method, however, the resolving power of the detector is kept different at the higher and lower portions of the measuring range.